How about a plastic tent like a boat cover. They keep a boat warmer like a green house.

You can't really design for -20° practically. What you need to do in that case is:

Dress warmer
Add some supplemental electric heaters
add as much insulation as you can... even on top of the deck!
figure out some way to store heat (heat sink) This means mass.

The supplemental strategies are not seaworthy and must be considered as dockside only. I know that the live aboards in New Rochelle all use propane, tanks on the dock and several have heaters of some sort installed over a deckhatch. Having the bottle on the dock makes it safer and more manageable (moving heavy bottles onto the deck in winter ain't fun).

I don't know that much about the boat side of heating but I've spent my whole life heating and cooling homes. I can't imagine that there would be anything better then a water source heat pump. You already have a forced air conditioning system. To make it a heat pump requires about $400 in additional components max. When connected to shore power it would be more efficient then heating with natural gas and twice as efficient as propane (lp). It would be about three times more cost effective as diesel. If offshore you would have to use your generator to power it but you would still get as much heating efficiency as if burning straight diesel in a heater.

I would be supprised if it wasn't a standard upgrade option with most marine a/c systems.

Yeah, I do tend to agree. The true heat (read like sitting in the sunshine in the summer) comes from the stove radiating at you. Too far across the room, and it gets a little drafty. I guess once I get this cat, I'll have to work on this problem. It sure will be a lot more tricky than in a mono.

... I forsee a central 40,000 BTU forced air heating and A/C system in Sean's future . Now that would make a dent in the problem.

Why the more I think about it the more I think his 'Admiral' should make this his Valentine's present.

A heat pump only works when the sea water temp is > 40F or so. When it's winter they don't work.

On our cat, I heat it at the dock with 2 x 1500 W heaters; one in each hull. But that's for working aboard, not living aboard and it is not enough Watts on a day near 32F / 0C.

My plan for living aboard is a singleDickinson "Antarctic" in one hull, a hot water coil inside the heater, hose to the opposite hull, and a small fan forced radiator. And a circulating pump. The computer fan on the radiator won't draw much, but the circ pump will be at least 1 A. The saloon is the warmest part of the boat from solar heating during the day. A fan blowing warm air from the saloon into the hulls isn't very workable IMO. This is a much simpler, cheaper, and more electrically efficient system than an Espar/Webasto system.

Friends of ours live aboard at a dock in Annapolis, MD on a St Francis 44. They use oil filled radiators in the hulls and move them around from saloon to bedrooms as the need arises. They don't try to heat the whole boat.

Insulation is very important in preventing condensation inside lockers that will ruin clothes, food, books and gear. We lived aboard in Annapolis for 3 winters and every year more lockers got insulation (closed cell foam glued to the hull mostly)

... My plan for living aboard is a singleDickinson "Antarctic" in one hull, a hot water coil inside the heater, hose to the opposite hull, and a small fan forced radiator. And a circulating pump. The computer fan on the radiator won't draw much, but the circ pump will be at least 1 A. The saloon is the warmest part of the boat from solar heating during the day. A fan blowing warm air from the saloon into the hulls isn't very workable IMO. This is a much simpler, cheaper, and more electrically efficient system than an Espar/Webasto system...

I wouldn’t expect a typical Computer “case” Fan to be effective as a (Fan-Forced) Heating Radiator driver.
Designed to cool computers, which only generate about 100 Watts (± 350 Btu/H) of heat, a computer fan may deliver between 25 - 150 cfm (at very high velocities).

__________________
Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

I've owned a Seawind 1000 for 5 years cruising mostly in the Gulf Coast area (TX, MS, AL and FL). Lowest temps are in the high 20's and usually much warmer but I've found a couple of catalytic propane heaters work great. I started with some small Colemans but have now switched to higher btu units. Only downside is that you need small propane bottles or you need to buy a hose converter for a larger tank. As long as you cruise propane friendly areas there shouldn't be a problem. I did install CO detectors as a saftety measure but have never had any problem with the catalytic style heater. The only downside is that it's difficult to maintain a temp band (settings are high, medium or low).

we use 1 or 2 webasto hot air heaters depending on the climate you are in
1 x 5 Kw unit will heat a 40 ft plus cat , in this case the unit is mounted under the saloon seat and the air is ducted into both hulls and in case 2 units are used they are mounted in each hull
we use air heaters for obvious reason the low weight !!!

I use an indoor propane heater. It has several automatic cutoff features to assure saftey. I combined it with one of the Kevlar propane bottles with a hose adapter and I am good to go. I put it in the salon during the day. At night I shift it to the hull with the entance to our cabin. Keeps us very toasty. At raises the moisture content inside, but it get so dry during the cold spells its not too much of an issue. Just have to wipe things down with a towel every now and again.